The present invention relates to a method in consolidation and drying of paper. A wet web is contacted with superheated steam for the purpose of reducing, by heat transfer from the superheated steam, the water content of the web by evaporation of a substantial part of the water.
Drying of a web-shaped material, such as paper, usually occurs by the wet web being contacted with hot surfaces or by blowing a hot gas, preferably air, towards the web.
In cylinder drying machines, the web-shaped material is heated by heated cylinders against which the web-shaped material is pressed by the web tension or by means of a felt or wire. The cylinders are heated by steam being supplied to their interior and condensing when contacting the casing of the cylinders. The condensate is evacuated continuously.
Water is preferably drawn off in free ventilated draws between the cylinders. The ventilation air, or at least the major thereof, is drawn off from the casing which encloses the cylinder drying machine. The amount of ventilation air drawn off is adjusted so that the moisture content of the exhaust air is adjusted towards a desired value, which is as high as possible in consideration of the risk of condensation etc.
Drying of so-called soft tissue occurs also on a single heated cylinder, a so-called yankee drier, in which drying occurs in combination with impingement of hot air or combustion gases.
Drying of paper also occurs completely without contact with a heat-emitting surface in drying machines with a so-called airborne web. In these drying machines, the web floats in a fixed position above or between blow boxes. The air leaving the blow boxes serves to carry the web, heat it and remove evaporated water.
The drying method affects to a very high degree the quality of the paper produced. The quality is also affected by a plurality of other parameters, such as how quick the paper is dried and in which temperature range it is treated.
Another method was already suggested in 1952 by Julian Dungler in U.S. Pat. No. 2,590,849. This method discloses a method for drying of cloth, paper and similar fibrous materials. Instead of using hot air or some other hot gas in drying, jets of superheated steam are directed at high speed towards the goods to be dried.
In conventional drying it is not possible to increase the temperature of the goods to be dried above the wet temperature as long as xe2x80x9cfree waterxe2x80x9d exists. By blowing steam, a bulk temperature corresponding to the current boiling point can be achieved very quickly during simultaneous condensation of part of supplied steam.
At the beginning of a paper drying process, where air at atmospheric pressure is used, a web has a temperature of only 50-60xc2x0 C. whereas, when blowing superheated steam, in steam atmosphere, the web quickly reaches a temperature between 90 and 100xc2x0 C. In addition to a reduction of the space required, this may result in an increased paper quality by the fibres being softer and being more strongly bonded to each other.
The advantages of steam drying appear to be so obvious that it is strange that this method did not gain a considerable market share, but since this has not occurred, there must have been decisive difficulties, so that the advantages did not outweigh these additional problems.
A main object of the present invention is to provide a simple and space-saving method for drying of paper.
A second object of the present invention is to provide a method of drying paper in an oxidation-free atmosphere.
A third object of the present invention is to provide a method for fixed drying of paper.
A fourth object of the present invention is to provide a method for drying of paper with superheated steam instead of e.g. air, in which expected quality gains can be achieved without increased costs of installation and operation.
The present invention relates to a method in consolidation and drying of paper. A wet web is contacted with superheated steam, whose temperature is 200-600xc2x0 C., for the purpose of reducing, by heat transfer from the super-heated steam, the water content of the web by evaporation of a substantial part of the water in the same.
In the suggested method, the wet web is supported and transported with a heat-conductive, gas-impermeable belt whose width is equal to or exceeds the width of the web and jets of the superheated steam are directed towards the side of the web facing away from the belt.
Properties and quality of a dried web are largely determined by the way of drying it. In addition to the technical conditions that must be taken into consideration, it must also be possible to produce the product economically to allow it to be sold in large volumes on a competitive market.
According to the present invention, a wet paper web is consolidated and dried in an oxidation-free atmosphere by contacting the web with superheated steam, whose temperature is 200-600xc2x0 C. The wet web is supported and transported with a heat-conductive, gas-impermeable belt whose width is equal to or exceeds the width of the web and jets of the superheated steam are directed towards the side of the web facing away from the belt. By heat-conductive belt is here and henceforth meant a belt whose thermal conductivity is so good as to function as a temperature-equalising factor in the contact with the web. The belt is conveniently made of metal, such as stainless steel, but polymers are not excluded. By jets is meant flows with high speed and of essentially arbitrary cross-section, such as circular and elongate.
The method according to the invention aims at providing quicker drying than conventional methods, but all the same affording reasonable time, essentially exceeding the time in, for example, the nip of the impulse drying, for consolidation of the web. Superheated steam is blown towards the web for a period of, for example, 0.5-5 s.
A wire may be arranged between the belt and the web, but preferably the web is in direct contact with the belt. The belt can also be prepared for varying adhesion, and in case of increased adhesion shrinkage in the transverse direction can be prevented.
The drying can occur in a single step by steam of the same temperature being supplied during the entire drying procedure, but it is in many cases advantageous to divide the drying into two or more steps. Then the temperature of the steam in a first step is suitably higher than in one or more subsequent steps.
In two-step drying, for instance the temperature of the steam in the first stage is selected to be between 400 and 600xc2x0 C., preferably between 480 and 540xc2x0 C., and in a second step, for example, a temperature between 300 and 400xc2x0 C. is selected, preferably between 330 and 370xc2x0 C. The speed of impingement should be above 50 m/s, preferably between 80 and 120 m/s in the first step and between 60 and 120 m/s in the second step.
Advantageously the web is preheated to 90-100xc2x0 C. before drying. This can conveniently take place by direct or indirect contact with essentially saturated water vapour. The preheating can be preceded by a treatment which serves to remove the air which is entrained in the wet web. The steam for preheating is suitably taken from the drying machine as evacuated steam from one of the drying steps, in the first place the second step or a later step in a multistage drying machine. The evacuated steam from the first drying step can also advantageously be used for supply to the second step or to a later step etc.
After preheating, the web can advantageously be mechanically dewatered to reduce the water content and simultaneously any remaining air can be evacuated. By this occurring at an increased temperature, the pressing will be more efficient and less water need be evaporated during drying. In this manner, it is also possible to render the wet pressing at the end of the wet end more efficient and thus reduce the investment cost by excluding one or more pairs of rolls. Analogously, it is advantageously possible to incorporate glazing in the drying machine where the web is already surrounded by hot water vapour.
According to a variant of the invention, the drying machine can besides be used as a chemical reactor by chemicals which affect the properties of the paper being supplied together with the vapour or in some other suitable manner in the preheating step and/or in one or more drying steps.